The invention relates to apparatus for processing plastic materials, and more particularly to improvements in extruders and like apparatus wherein a continuous stream of plastic material is caused to advance to one or more dies or mold cavities to be converted into a succession of discrete products or into one or more continuous products in the form of strands, sheets, rods, bars or the like.
German Pat. No. 29 42 849 to Kreyenborg discloses an apparatus wherein the flow of plastic material to a die or the like can be interrupted by a reciprocable holder for a series of filters in discrete compartments. The filters can be moved into register with the passage so that the plastic material must penetrate through a selected filter in order to be relieved of foreign matter on its way toward the shaping station. The arrangement is such that some plastic material can penetrate into a next-following filter compartment while the preceding filter compartment establishes communication between the inlet and outlet ends of the passage for plasticized material. The next-following filter compartment is then connected with the atmosphere by a suitable valve so that it can be filled with plasticized material before it reaches the operative position of alignment with the passage. The patented apparatus operates satisfactorily for the processing of unfoamed plastic materials.
German Auslegeschrift No. 2 153 962 of Buchheit et al. discloses an apparatus wherein the leader of a stream of plasticized material is permitted to escape by way of a bypass channel before the passage for the flow of plasticized material to the shaping station is traversed by a filter. This ensures that the improperly heated and/or air-containing and/or otherwise unacceptable portion of the stream of plasticized material is not converted into an unsatisfactory product. Moreover, it is possible to gain access to a filter while the plasticized material is caused to penetrate through a spare filter or into the aforementioned bypass channel.
German Offenlegungsschrift No. 2 130 501 of block-shaped filter holders can be advanced into register with the path for the flow of plasticized material toward the shaping station. The holder which is maintained in a position of readiness can be filled with plasticized material in response to opening of a valve so that the plasticized material expels air before the holder advances to a position in which the plasticized material flowing toward the shaping station must penetrate through its filter or filters.
A drawback of the aforediscussed apparatus is that removal of a clogged filter can present problems, particularly injuries to attendants and/or contamination of the surrounding area, if the material to be converted into shaped plastic articles is a foamed thermoplastic substance which is maintained at an elevated pressure during flow toward the shaping station. Foaming is achieved by mixing the plasticized material with a suitable blowing or foaming agent which causes the plasticized material to expand upstream and/or downstream of the filter. The filter compartment which is in register with the passage for the flow of plasticized material to the shaping station (e.g., to a die with apertures for strands of foamed plastic material) is filled with plasticized material which is maintained at an elevated pressure. Thus, the contents of the filter compartment expand abruptly as soon as the compartment is permitted to communicate with the surrounding atmosphere. The material which is thereby expelled from the filter compartment can contaminate the area in and around the apparatus and can cause serious injury or injuries to the attendant or attendants, especially since the material leaving the filter compartment is normally maintained at an elevated temperature.